1. Field of the Invention
This invention relates to devices and methods for moving an implant in a body, and more particularly to such devices and methods that apply pushing forces with a flexible attachment and that magnetically steer the implant in the body with high accuracy.
2. Description of Related Art
There is a large body of conventional (nonmagnetic) stereotactic prior art, in which a frame (e.g., a so-called xe2x80x9cBRW Framexe2x80x9d) is attached to the skull to provide a navigation framework. Such a frame has arcs to determine an angle of an xe2x80x9cinsertion guidexe2x80x9d which is usually a straight tube through which some medical therapeutic agent is passed, such as a biopsy tool. These methods have been confined to straightline approaches to a target.
There is also a smaller body of prior art in which a handheld permanent magnet or an electromagnet is used to move a metallic implant.
Previous implants for delivering medication or therapy to body tissues, and particularly brain tissue, have generally relied upon the navigation of tethered implants within vessels, or navigation of tethered or untethered implants moved intraparenchymally (in general brain tissue) by magnetic force.
Navigation of untethered implants, in the past, has generally comprised finding ways to apply magnetic force optimally, including both magnitude and direction, for a given step of xe2x80x9cfreexe2x80x9d motion. However, difficulty in finding a set of currents to accomplish a move step is encountered because of the complexity of calculating the magnetic forces resulting from multiple coils.
It is well-known that two like coils on a common axis, having like currents, provide a highly uniform magnetic field on their axis at the midpoint between them. In addition, it is known that the field is approximately uniform for an appreciable region around the midpoint, and relatively strong, as compared with any other two-coil arrangement having the same coil currents. This arrangement of coils and currents meets the need for an accurate, strong guiding torque applied to a magnetic implant near the midpoint between the coils. Because the field is quite uniform near the midpoint, undesired magnetic forces on the implant are negligible. However, this arrangement is less suitable for a moving implant when the implant is some distance from the midpoint between the coils or not on the axis, or when the implant axis is not along the coil axis. In these important cases, this simple coil arrangement cannot provide accurate directional guidance. Furthermore, simple vector combinations of three such coil pair arrangements cannot provide accurate guidance in an arbitrary direction, except at one spot at the center of the arrangement.
The Magnetic Stereotaxis System (MSS) originated from the hopes that a less-invasive methodology could be developed which would allow neurosurgeons to operate in previously inaccessible regions of the brain. By introducing a small permanent magnetic implant into the brain through a small xe2x80x9cburr holexe2x80x9d drilled through the skull prior to the operation, large superconducting coils could be used in conjunction with a pushing mechanism to magnetically guide the implant and overlaying catheter through the brain""s parenchyma, all the while avoiding the important structures of the brain. The operational methodology of the MSS was, and continues to be, expected to be less destructive to the tissues of the brain than the shunts, straight tubes, and other devices associated with conventional techniques in neurosurgery.
The first MSS was conceptually developed in 1984 as the Video Tumor Fighter (VTF), and is shown in U.S. Pat. No. 4,869,247 issued Sep. 26, 1989. This system specifically focused on the eradication of deep-seated brain tumors via hyperthermia-based treatment. It was envisioned that the magnetic coils of the VTF would guide a small (xcx9c3 mm diameter) magnetic thermosphere through the brain into a tumor. Rastering the implant throughout the volume of the growth, the tumor cells could be destroyed by inductively heating the implant with radio-frequency radiation.
Further studies revealed that the reality of a magnetomotive based system used to direct a small implant promised numerous applications other than the hyperthermia-based treatment of brain tumors by induction. These included: biopsy, pallidotomy, delivery of precision radiation therapy, magnetically placed implants that deliver chemotherapy to otherwise inaccessible tumor locations, and (by attaching a semi-permeable catheter to the implant) the delivery of chemicals to specific sites in the brain without the need for penetrating the blood-brain barrier which has complicated contemporary systemic chemical delivery. This means of chemical delivery seemed particularly hopeful in the treatment of Parkinson""s disease, where the catheter could be used to deliver dopamine to the affected regions of the brain with minimal indiscriminate distribution of the neurotransmitter to the surrounding tissue, thereby lessening attendant side effects. It was in the light of these possible broadened applications of the VTF that the system became known as the MSS.
Referring now to FIG. 1A and FIG. 1B, the most recent MSS apparatus 10 included six superconducting coils (not visible in FIG. 1A and FIG. 1B) located in a rectangular box or helmet 12. With the z-axis defined in the direction of the axial component of the head, the x- and y-coil axes are rotated 45xc2x0 from the sagittal plane 14 of the head, which would be positioned in opening 16. The x- and y-coil axes are symmetrically located such that the horizontal extensions 22 of the MSS apparatus 10 away from the patient""s body is minimized. Because the lower edge of the treatable part of the brain is typically located 10 cm above the shoulder line for an average adult, the z-coils (located on the body-axis of the supine patient) were compressed to allow for a maximum extension of the head into helmet 12.
The vision component of the MSS consists of a superposition of pre-operative MRI images referenced by biplanar fluoroscopy cameras 20 linked to a real-time host system (not shown in FIG. 1A and FIG. 1B). Both cameras 20 are calibrated to the MSS six-coil helmet design. X-ray generators for cameras 20 are located inside magnetic shields 22. Using x-ray visible fiducial markers located on the skull of the conscious patient, the coordination of the implant""s position inside the cranial volume to the helmet""s reference system (and hence the corresponding preoperative MRI scan) is done through a series of coordinate transformations executed by a host system and displayed for the surgeon on a workstation.
The central problem to the inductively-based guidance of a magnetic implant pertains to the inverse problem of electromagnetism as influenced by Earnshaw""s theorem. The conventional problem of electromagnetism centers on the evaluation of the gradient and magnetic field given established magnetomotive sources. For the MSS, however, the situation is reversed in that the magnetic field and its gradient are specified at a point in space while the strengths of the six actuators are to be determined. Control of the motion and position of an untethered implant would be difficult in the MSS, given the fundamental instability of a non-diamagnetic moment in a static or quasi-static magnetic field as related to Earnshaw""s theorem for static/quasi-static magnetic fields, if it were not for the resistive nature of the parenchyma. In early tests, small cylindrical (up to 5 mm in length and 5 mm in diameter) permanently magnetized NdBFe objects were used. The relatively strong moment of these objects (0.016 A-m2 to more than 0.04 A-m2) facilitated the creation of the necessary aligning torque without the requirement of a strong magnetizing field, resulting in lower current values.
The permanent magnetization of the implant requires a predetermined magnetic field in order to ensure that the implant is oriented in the desired direction. While it is possible to generate a magnetic force to displace the implant, it was found that the requirement of specific force and field alignment could result in unobtainable currents (as high as thousands of amperes). It was also found that even for viable solutions, the equilibrium state was sometimes unstable to such an extent that the implant tended to be difficult to control.
The invention is an apparatus and method for moving an implant in the body by applying pushing forces with a flexible attachment and magnetically steering the implant in the body with high accuracy and controllability. Because the intended moving force is applied non-magnetically, it is possible and desirable to apply currents in the magnetic steering apparatus in such combinations as to maximize the magnetic field at a body location inside the coil array to thereby provide optimal directional guidance torque on an implant while minimizing undesired translational force on the implant.
According to one aspect of the invention, there is provided a method for controlling movement of a catheter through a medium, in which a flexible catheter having a magnetic tip is pushed through a medium, and a magnetic field having a magnitude and orientation effective to guide the mechanically-pushed catheter tip in a predetermined direction is applied.
According to another aspect of the invention, a method for providing stepwise movement of a catheter having a magnetic tip is provided, in which the method includes the steps of selecting a desired path of the catheter through living tissue, inserting the catheter tip into the living tissue, determining actual positions of the magnetic tip and correction vectors (the correction vectors representing differences between locations on the desired path and the actual positions of the magnetic tip), storing values of correction vectors in a memory, and applying a magnetic field adjusted to achieve movement of the magnetic tip at least approximately along the desired path, the adjustment depending upon at least one stored set of values of correction vectors.
Also provided is a device for guiding a catheter having a magnetic tip through a medium, the device comprising a helmet having a cavity configured to encompass a medium through which a catheter is to be guided, a magnetic field generator generating a magnetic field within the cavity, a position sensor sensing a location of a magnetic tip of a catheter in the cavity and generating a signal indicative of the sensed location, an advancement mechanism pushing the magnetic tip of the catheter through the medium, and a processor responsive to the signal from the position sensor and having an operator control input, the processor being configured to control the magnetic field generated by the magnetic field generator in response to commands input via the operator control input and the signal received from the position sensor.
The above embodiments may also incorporate significant additional improvements, including, for example, the minimization of a current metric, so that the proper magnetic field to guide the magnetic tip through the medium is generated with a near-minimum amount of current.
The methods and apparatuses of this invention provide the ability to more accurately direct a seed or catheter in the brain or other parts of the body, including the path to that position. Highly accurate directional guidance of implants is possible over arbitrary nonlinear paths, and the implant can be guided freely through tissues such as brain tissue, without being limited to the interior of vessels.
Additional advantages of the present invention over prior art systems are that:
(1) Solutions applicable to guiding an implant on a predetermined path are simpler, and thus, are found more rapidly and with less likelihood of error for a given step of motion.
(2) Solutions are much more stable than with prior art systems, and are free of runaway conditions.
(3) Currents applied by the new method are generally considerably smaller than with previous methods; therefore, the current changes between steps are smaller, allowing changes to be made much more rapidly and accurately between steps, and with less possibility of quenching superconducting magnets.
(4) Guidance force occurs without skid, which is a condition in which the magnetic field that orients the implant and the magnetic force are in different directions so that the axis of the implant skids along the path.
(5) Currents are applied in a simple temporal fashion, moving directly from one set to another set between two steps of motion. The actual force impulse causing each step of motion is from the duration and distance of the externally applied non-magnetic force during that step. (Prior art systems ramped currents from conditions for subthreshold force to that of a moving force and then back down below threshold at the appropriate time, which is a complex dynamic sequence subject to substantial error in step length due to the tribological nature of the implant and tissue.
(6) Navigation can now occur continuously rather than in steps.
It is thus an object of the invention to provide a method for controlling the motion of a catheter in any predetermined direction.
It is a further object of the invention to control the motion of a catheter by applying a torque to the catheter to guide its direction with a reliable, predictable strength.
It is yet another object of the invention to control the motion of a catheter rapidly, accurately, and reliably, even when the magnetic system used in conjunction with the catheter includes superconducting coils that are vulnerable to misoperation from too rapid current changes.